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The Advancement and Superiority of Longitudinal Submerged Arc Welded Pipe

Date:2021-01-19    keywords:Longitudinal submerged arc welded pipe, LSAW
The vast majority of oil and gas pipelines in the world require the use of longitudinal submerged arc welded pipes (LSAW). Canada and other western developed countries have improved the technology of spiral welded pipe (SSAW), such as pre-welding, precision welding, multi-wire welding, mechanical expansion, etc., which are designed with reference to the LSAW steel pipe process, which shows that the straight seam The advanced nature of welding pipe technology. Specifically:

1. Technically speaking, the advantages of LSAW steel pipe:

① The base material of the longitudinally welded pipe is a single controlled-rolled steel plate with isotropic properties, which can be 100% non-destructive testing;
②The stress state of the longitudinally welded pipe is good after forming. Due to the mechanical expansion, the internal stress can be eliminated and the geometrical accuracy of the steel pipe can be improved;
③The internal quality of the straight seam welded pipe is good, because the welding is carried out on the plane, the welding conditions, automatic tracking, and non-destructive testing are easy to carry out, which makes the weld height low and the internal and external welds high precision;
④Longitudinal welded pipe has a wide range of wall thickness, generally 6-25 mm, and the maximum thickness can reach 45 mm;
⑤Longitudinal welded pipe has good bending and flattening resistance;
⑥Longitudinal welded pipe welds are 60% shorter than spiral welded pipes, and the probability of weld defects is low;
⑦High reliability of longitudinal welded pipe.

According to statistics, the failure rate of LSAW gas transmission in foreign countries is 0.05~1.44 times/103 kilometers in 50 years, and the failure rate is relatively low. Therefore, it is suitable for application in the second, third and fourth category areas.


  
2. From the perspective of being suitable for marine oil and gas transportation, it is bound to focus on the construction of LSAW.
At present, a considerable part of the world's oil and gas resources are buried in the ocean. From the perspective of future natural gas development, it is mainly buried in the ocean in the form of "combustible ice". At present, it is recognized that the total energy of "combustible ice" in the world is two to three times the total of all coal, oil, and natural gas in the world. This is also the case in my country, which will be the main source of energy in the future.
 
In the near term, my country is accelerating the pace of oil and gas development in the East China Sea. With the development of oil exploration to the depths of the ocean, due to the impact of pressure, impact and bending forces on the pipelines laying on the seabed, flattening occurs. It is the weak link of spiral welded pipe (SSAW). In order to improve pipeline transportation capacity and ensure safety, submarine pipelines are developing in the direction of thick wall, so submarine pipelines mostly use straight seam welded pipes.

From another point of view, because it is difficult to install butt welding of pipelines in the ocean, the steel pipes are required to have high ovality, moderateness, bevel protection, and cross-sectional errors. Otherwise, the work efficiency will be seriously affected. As a result, LSAW pipes are more accurate than spiral welded pipes (SSAW), and repair welding is also easy. Therefore, from the perspective of requirements in this regard, it is also inclined to use LSAW pipes.
 
3. Straight seam welded pipes are required for coal slurry and slurry pipelines.
The pipeline transportation of coal and mineral powder in a "paste" state is an effective international upward direction. Because the internal weld height of the spiral welded pipe (SSAW) is a spiral line, the resistance to coal slurry is relatively large, and the pipeline needs a large wall thickness Tend to use straight seam welded pipe (LSAW).
 
4. Longitudinal welded pipes are required for machinery, construction and chemical industry.
The valve seat of the machinery industry manufactures the valve seat. At present, the inner hole is processed by forging parts, which is time-consuming and costly. Use straight seam welded pipe; chemical pipe also hope to use straight seam welded pipe.

Straight seam steel pipe inspection:

1) Defect detection of delamination of steel plate

The flaw detection of delamination of steel plates is different from that conducted by steel plate manufacturers abroad and responsible for ensuring the quality requirements of users. In foreign countries, the steel pipe factory and the steel plate factory are usually the same large-scale joint enterprise, and the steel plate factory conducts the flaw detection of the delamination defects of the steel plate. Moreover, advanced foreign instruments are fully capable of achieving 100% coverage for delamination of steel plates. Different from foreign forms, in the domestic LSAW steel pipe production, the steel plate delamination defect detection is carried out by the steel pipe factory itself. According to the original design, a scan can only completely cover the edge of the steel plate, and the coverage of the entire plate can reach more than 25%. To achieve 100% coverage of the delamination detection of steel plates, it is necessary to perform back and forth scans. In this regard, there is a certain gap between domestic and foreign countries.

2) Flaw detection of welds

Foreign LSAW steel pipe manufacturers attach great importance to and rely heavily on ultrasonic inspection of welds. Generally, ultrasonic inspection of the full length of the welds is taken as the internal quality control of the factory. At the same time, X-ray industrial television inspection equipment is also set up, and its static sensitivity is also high (less than 16%). It is generally used to re-inspect the defect parts of the ultrasonic inspection before the expansion. Only a few manufacturers conduct the steel pipe before the expansion. Full weld ultrasonic flaw detection and X-ray industrial TV inspection. After the expansion and water pressure are completed, the automatic ultrasonic flaw detection of the full weld is mainly used. At the same time, the ultrasonic hand probe inspection and the tube end film inspection in the automatic blind area of the weld at both ends of the steel pipe are added.


Domestic LSAW steel pipe production plants perform ultrasonic flaw detection and X-ray industrial television flaw detection on 100% of the full length of the weld before and after water pressure. The flaw detection before hydraulic pressure is used for internal quality control of the factory, and the flaw detection after hydraulic pressure is used for quality assurance and acceptance inspection. Except for a few steel pipe factories, most domestic LSAW steel pipe manufacturers rely on X-ray industrial television inspection of welds, but the sensitivity of domestic X-ray industrial television inspection equipment is not very high. In contrast, domestic flaw detection and inspection of steel pipe welds are more dependent on the quality of the operators' work, while foreign factories' inspection of welds is more dependent on the equipment itself, and the reliability is higher.


Tips: Straight seam steel pipe is a steel pipe whose weld seam is parallel to the longitudinal direction of the steel pipe. Straight seam steel pipes can be divided into high-frequency straight seam steel pipes (ERW Pipe) and submerged arc welded straight seam steel pipes (LSAW) according to the production process. Straight seam welded pipe has simple production process, high production efficiency, low cost and rapid development.


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